1. Field of the Invention
The present invention relates to a shading corrector apparatus compensating for shading on image signals produced from a solid-state image sensor having an array of photo-sensors, each of which has a set of photosensitive cells different in sensitivity from each other.
2. Description of the Background Art
As known in the art, a solid-state image sensor of the type is available which has its dynamic range narrower with respect to intensity of light incident thereto. The dynamic range has its upper limit defined by the output level of the individual photosensitive cell saturating when input light intensity exceeds a preselected value, and has its lower limit restricted by the S/N (Signal-to-Noise) ratio of a signal derived from incident light increasing above a preselected value. It follows that if the sensitivity of the photosensitive cell is increased to broaden the dynamic range, then saturation is likely to occur so as to prevent an adequate amount of light from being attained. To broaden the dynamic range also, if the sensitivity of the photosensitive cell is lowered, then the noise level remains constant without regard to sensitivity so that the lower limit of the dynamic range has to rise accordingly to a decrease in S/N ratio. In any case, therefore, the dynamic range becomes narrowed.
In light of the above, Japanese patent laid-open publication No. 207376/1993 discloses a solid-state image pickup apparatus having photodiodes, each of which is divided into a plurality of regions, or photosensitive cells. In this image pickup apparatus, a switching device is assigned to each of the photodiodes to switch a photosensitive area generating a signal charge which is to be stored in the capacitive element of a unit pixel, thereby establishing the switchable sensitivity of the photosensitive array. In this configuration, even where the capacitance of the capacitor, which is decisive over the sensitivity, is reduced in order to increase the sensitivity, the reduced photosensitive area can lower the sensitivity for picking up a relatively lighter subject, thereby obviating the saturation within a controllable range of integration period of time. For picking up a relatively darker subject, the photosensitive area can be increased to increase the sensitivity, thereby accomplishing the operation within a shorter integration period of time. Consequently, the image pickup apparatus taught by the above document successfully broadens the dynamic range.
Japanese patent laid-open publication No. 205589/1997 teaches a solid-state image pickup apparatus in which each of the photo-sensors is segmented into a plurality of photosensitive regions, or photosensitive cells, different in sensitivity from each other. Signal charges read out from the photosensitive regions of nearby photo-sensors having the same sensitivity are mixed by a vertical transfer register and then transferred in the vertical direction of the photosensitive array. At the same time, signal charges read out from the photosensitive regions different in sensitivity are transferred by a plurality of horizontal transfer registers independently of each other. Further, from the photosensitive region or regions at least except those with the lowest sensitivity, the signal charges, or signals based thereon, are derived to be clipped and mixed with or added to the signal charges, or signals based thereon, outputted from the remaining photosensitive region or regions including those with the lowest sensitivity. Consequently, signal charges, or signals based thereon, at the higher sensitivity side are clamped at the same clamp level, so that the dynamic range is broadened without generating fixed-pattern noise ascribable to irregularity in saturation level of the pixels.
Generally, in a solid-state image pickup apparatus, image signals outputted from a solid-state image sensor may involve shading due to, e.g. irregularity in quantity of light incident to the respective photosensitive cells. In the case of a digital camera of the type including microlenses formed on the photosensitive array of its image sensor, the quantity of light incident to the respective photosensitive cells noticeably varies in dependence upon the direction of the incidence particular to the microlenses. More specifically, to a photosensitive cell located in the vicinity of the edge of the imaging frame of the image sensor, light is incident often inclined, so that the incident light is poorer in quantity than one incident to a photosensitive cell located near the center of the frame. Consequently, luminance of the signals produced in the vicinity of the edge of the frame is poorer, thus resulting in shading.
It is a common practice with a digital camera of the type described above to use, e.g. digital processing and a memory to correct image signals involving shading, thereby simplifying a shading correcting circuit. Shading is generally classified into black shading ascribable to an image pickup tube or CCDs (Charge-Coupled Devices) and white shading ascribable to a prism or a lens included in optics. To correct, e.g. black shading, it has been customary to store in a buffer memory black shading data generated with no incident light received and then subtract, during usual imaging operation, the output of the buffer memory from the image signals produced the imaging operation.
The conventional solid-state image pickup apparatus described above mixes signal charges outputted from a set of photosensitive cells, different in sensitivity from each other, of each pixel, thereby producing pixel-by-pixel image signals over a broad dynamic range. However, the shading of the image signals cannot adequately be compensated for because the photoelectric transduction characteristic and the incidence angle of light, for example, differ from one to another photosensitive cell.